A spatial light modulator is known as a device for actively controlling a phase and the like of light.
A spatial light modulator of an optical address type is disclosed in U.S. Pat. No. 5,841,489 and Japanese Unexamined Patent Application Publication No.6-51340, for example. This spatial light modulator has: an address portion, a read portion, and a reflecting mirror provided between the address portion and the read portion. The address portion has a photoconductive layer. The read portion has: an optical modulation portion having a liquid crystal cell, and a light transmission portion for transmitting read light therethrough. The light transmission portion has a glass substrate and the like for transmitting the read light therethrough. The reflecting mirror is constituted by a dielectric multilayer film in which SiO2 and TiO2 are alternately deposited one on another. The voltage applied through the liquid crystal cell is changed in accordance with address light falling incident on the photoconductive layer. As a result, molecular arrangement of the liquid crystal changes and the refractive index of the liquid crystal also changes. When the read light passes through the liquid crystal cell, the phase of the read light is modulated.
This spatial light modulator is of a reflection type. Many spatial light modulators of a transmission type employ a metal mesh as an electrode. In the spatial light modulator of a transmission type, a portion on which a shadow of the metal mesh is cast can not be used, and therefore the whole screen can not be effectively used. On the other hand, the spatial light modulator of a reflection type does not have such a problem, and the whole screen can be effectively used.
Conventionally, the spatial light modulator of a reflection type has normally been used to control the phase of a laser beam of a single wavelength. However, it has been proposed by Itoh et al. in “Reflect-Type, Highpower-Laser Pulse Shaping System and its Operation Verified by Frequency Resolved Optical Gating”, Laser Research, Vol. 28, No. 8, pp. 511 to 515, August 2000 that the spatial light modulator of a reflection type is used to control the phase of an ultrashort light pulse having a wide spectrum to shape the waveform of the ultrashort light pulse.